1. Field of the Invention
The present invention relates to an optical system that is suitably used for an image pickup apparatus such as a digital still camera or a video camera using a solid-state image pickup element or a camera using a silver salt film.
2. Description of the Related Art
As an image pickup optical system of a long focal length, a so-called telephoto-type image pickup optical system (a telephoto lens) that is, in order from an object side to an image side, configured by a front lens unit having a positive refractive power and a rear lens unit having a negative refractive power is known. The long focal length, for example, means a focal length that is longer than a size of an effective image pickup range. Commonly, in the telephoto lens having a long focal length, a total lens length becomes long as the foal length is extended. When a telephoto lens which has a small F-number (an aperture ratio) is obtained, especially a lens diameter of the rear lens unit is increased.
Therefore, a telephoto lens in which the front lens unit is appropriately configured so as to have a short total lens length is proposed. In Japanese Patent Laid-Open No. 63-194214, for example, a telephoto lens having a focal length of 100 mm, an F-number of 4.5, a telephoto ratio (a ratio of a total length with respect to a focal length) of 0.68 is disclosed. In Japanese Patent Laid-Open No. 63-106714, for example, a telephoto lens having the focal length of 300 mm, the F-number of 4.6, and the telephoto ratio of 0.70 is disclosed.
As the telephoto lens commonly has a longer focal length, a whole of the lens system is enlarged. In addition, when the telephoto lens having a small F-number is obtained, the whole of the lens diameter is increased. Therefore, it is important for the telephoto lens (especially, the telephoto lens having a small F-number with respect to the focal length) to reduce the size of the whole of the lens system.
Commonly, in the telephoto lens, as the F-number is decreased or the focal length is increased, particularly the size of the front lens unit having a positive refractive power is enlarged and also the weight of the front lens unit is increased. When the refractive power of each unit is simply strengthened in order to reduce the size of the front lens unit, an amount of variation of various aberrations is increased with respect to various kinds of manufacturing errors. In this case, the manufacture is difficult since surface accuracy or assembly accuracy that is required for each lens unit (especially the front lens unit), and as a result, it is difficult to obtain a desired optical performance. Therefore, when a small-size and lightweight telephoto lens has resistance to the manufacturing error is obtained, it is important to appropriately set a configuration of the front lens unit having a positive refractive power.
However, in the configuration of the front lens unit disclosed in Japanese Patent Laid-Open No. 63-194214 or Japanese Patent Laid-Open No. 63-106714, when the total lens length is further shortened, the amount of variation of the various aberrations with respect to the manufacturing error is increased. Therefore, in the configuration of Japanese Patent Laid-Open No. 63-194214 or Japanese Patent Laid-Open No. 63-106714, a lightweight telephoto lens which has resistance to the manufacturing error and has a short total length cannot be obtained.